1. Technical Field
This invention relates to a method for making norbornene polymers. Particularly, this invention relates to the ring-opening polymerization of norbornene monomers in the presence of a metathesis catalyst system. More particularly, the present invention pertains to the bulk ring-opening polymerization of a solution of norbornene monomers containing a pot life adjusting agent. This invention also relates to long pot life formulations for producing norbornene type polymers having a high thermal deformation temperature, a high modulus of elasticity, and improved impact strength.
2. State of the Art
The technique of bulk ring-opening polymerization of norbornene type monomers such as alkyl substituted norbornene, dicyclopentadiene and tricyclopentadiene by means of a molybdenum and/or tungsten compound metathesis catalyst within a mold is well known as disclosed in U.S. Pat. No. 4,380,617.
Generally, bulk ring-opening polymerization in a mold, e.g., reaction injection molding (RIM), is conducted by mixing a norbornene type monomer with a metathesis catalyst and cocatalyst (activating agent) in the absence of a solvent, conveying the reactive mixture to a mold, then letting the polymerization reaction take place to form a solid article. After mixing, polymerization commences and the viscosity of the reaction solution increases until passing the point where fluidity is almost lost. The time it takes the system to reach this point after mixing is called pot life. Stated differently, pot life is the time interval between the mixing of the reactive ingredients to form the reactive solution and the point where the solution becomes too viscous (e.g. gels) to adequately fill a mold. After this point the polymerization reaction progresses rapidly and the gel converts to a solid (e.g., a high conversion of monomer to polymer is achieved). An exotherm is associated with the rapid polymerization rate causing the unconverted monomer to vaporize when its boiling point is reached. The vaporization of unconverted monomer gives the system the appearance of smoking. Accordingly, the time interval it takes from mixing the reactants to the point at which the exotherm (e.g. smoke is produced) is attained is defined as the smoking time.
Early attempts at bulk polymerization produced reactions that were too rapid and, therefore, uncontrollable. The combination of molybdenum or tungsten catalyst and a cocatalyst such as alkylaluminum halide proved to be too active; progressing rapidly even at the room temperature. For this reason, methods have been proposed to prolong the pot life of bulk polymerizable reactive monomer formulations and to prevent premature polymerization. Approaches to improve catalyst systems by utilizing ether, ester, ketone, or nitrile activity adjusting agents have been proposed as disclosed in Japanese Patent Application Nos. 58-129013, 61-120814, and 61-179214.
However, in conventional techniques pot life is still inadequate, leading to operational difficulties in molding. The premature increase in viscosity of the reaction solution makes it difficult to uniformly convey the reaction solution throughout the mold, resulting in molded products with flow marks or weld lines that are generated by non-uniform polymerization. This leads to products of poor surface quality and reduced physical properties.
Moreover, in cases where a reinforcing material such as glass fiber is placed within the mold prior to the introduction of the reaction solution (to make fiber-reinforced products), viscous reaction solutions may cause the shifting of the reinforcing fiber within the mold or insufficient permeation of the reaction solution into the reinforcing fiber matrix.
In a further development, a method to improve pot life by using a tertiary alcohol in conjunction with an alkylaluminum or alkylaluminum halide cocatalyst has been proposed as disclosed in Japanese Patent Application No. 63-332776. Although pot life is prolonged by this method, smoking time is also prolonged along with it. In some cases, the polymerization activity is reduced, and a high conversion ratio can not be attained.
Tricyclic norbornene type monomers such as dicyclopentadiene (DCP) are widely used since they are industrially easy to obtain. However, when tricyclopentadiene (TCP) is used as a comonomer is raise the heat resistance of poly(DCP)resin, the pot life of the reaction solution is shortened. Furthermore, as the amount of TCP is increased, the impact resistance of the polymer product decreases.
In the bulk ring-opening polymerization of norbornene type monomers, the objective is to balance ease of operation, moldability, and productivity with the physical properties of the polymer product so obtained. In other words, premature polymerization of the reaction solution at the initial stages of the molding operation must be prevented. The reaction components and additives must be mixed completely, change of viscosity during the feeding of the solution into the mold must be curbed, and the solution must be polymerized immediately after feeding it into the mold. Moreover, the physical properties of the polymer product must be acceptable.
Therefore, it is important to control the reaction solution so that the pot life can be prolonged without overly prolonging the smoking time.